You don't? Really? Here are just a few of the differences you will find between battling the in-game AI and battling another person:
1. In-game AI Pokemon (outside of the Battle Frontier) do not have EV points.
2. In-game AI opponents RARELY will switch out if one of their Pokemon is inflicted with a status (i.e. Sleep, Burn, etc.). They will typically leave the affected Pokemon in until it faints.
3. In-game AI opponents RARELY have well-rounded teams. They typically all share a common weakness, especially all the gym trainers, the "elite" 4, and the especially the "champion".
4. In-game AI opponents will NEVER utilize specific strategies (i.e. Rest/Sleep Talk, Substitute/Focus Punch, Baton Pass, Spikes/Whirlwind, Curse/Rest, etc.) in order to win.
5. In-game AI Pokemon will not have a moveset that gives them the best type coverage.
6. In-game AI opponents will randomly attack using non-offensive moves, sometimes with moves that have absolutely no affect on your Pokemon (i.e. using Charge against a Swampert).
I am sure I could rattle off at least 10 more, but you get the idea. There is really a HUGE difference between battling the AI and battling against someone else. One of the many things I was hoping to see in DPP was a difficulty level that would at least make the game challenging for people older than 10. It just seems that they have forgotten that a lot of people that enjoy playing Pokemon have been around since RBY and really aren't 10 anymore.